In recent years, with the electric power shortage becoming more serious, rapid introduction of natural energy, such as wind power generation and photovoltaic power generation, and stabilization of electric power systems (e.g., maintenance of frequencies and voltages) have become issues to be addressed globally. As one of the measures to address the issues, installation of large-capacity storage batteries to achieve, for example, smoothing of output fluctuation, accumulation of surplus power, and load leveling has been receiving attention.
One of large-capacity storage batteries is a redox-flow battery (hereinafter, may be referred to as an “RF battery”). An RF battery is a battery which performs charging and discharging by using a positive electrode electrolyte and a negative electrode electrolyte, each containing metal ions (active material) whose valence is changed by oxidation-reduction. FIG. 7 shows an operating principle diagram of a vanadium-based RF battery 100 which uses a vanadium electrolyte containing V ions serving as an active material for each of the positive electrode electrolyte and the negative electrode electrolyte. In a battery cell 100C shown in FIG. 7, solid-line arrows and broken-line arrows indicate a charge reaction and a discharge reaction, respectively.
The RF battery 100 includes a battery cell 100C which is separated into a positive electrode cell 102 and a negative electrode cell 103 by a membrane 101 that allows hydrogen ions to permeate therethrough. The positive electrode cell 102 contains a positive electrode 104 and is connected via conduits 108 and 110 to a positive electrode electrolyte tank 106 that stores a positive electrode electrolyte. Similarly, the negative electrode cell 103 contains a negative electrode 105 and is connected via conduits 109 and 111 to a negative electrode electrolyte tank 107 that stores a negative electrode electrolyte. The electrolytes stored in the positive electrode electrolyte tank 106 and the negative electrode electrolyte tank 107 are circulated into the positive electrode cell 102 and the negative electrode cell 103 by pumps 112 and 113, respectively, during charging and discharging.
The battery cell 100C is usually formed inside a structure referred to as a cell stack 200 as shown in the lower view of FIG. 8. The cell stack 200 has a structure, as shown in the upper view of FIG. 8, in which a plurality of battery cells 100C are stacked, each battery cell 100C being formed by stacking a positive electrode 104, a membrane 101, and a negative electrode 105 and sandwiched between cell frames 120, each cell frame 120 including a picture frame-like frame body 122 and a bipolar plate 121 integrated therewith. That is, a battery cell 100C is formed between bipolar plates 121 of adjacent cell frames 120, and a positive electrode 104 (positive electrode cell 102) of a battery cell 100C and a negative electrode 105 (negative electrode cell 103) of an adjacent battery cell 100C are disposed with a bipolar plate 121 therebetween, on the front and back side of the bipolar plate 121. In this structure, gaps between the individual cell frames 120 are sealed with a sealing structure 127.
In the cell stack 200, circulation of electrolytes into battery cells 100C is performed by liquid supplying manifolds 123 and 124 and liquid discharging manifolds 125 and 126 formed on frame bodies 122. A positive electrode electrolyte is supplied from a liquid supplying manifold 123 through a groove formed on one surface side (front side of the sheet) of a frame body 122 to a positive electrode 104 disposed on a first surface side of a bipolar plate 121. The positive electrode electrolyte is discharged through a groove formed on the upper part of the frame body 122 to a liquid discharging manifold 125. Similarly, a negative electrode electrolyte is supplied from a liquid supplying manifold 124 through a groove formed on the other surface side (back side of the sheet) of the frame body 122 to a negative electrode 105 disposed on a second surface side of the bipolar plate 121. The negative electrode electrolyte is discharged through a groove formed on the upper part of the frame body 122 to a liquid discharging manifold 126.
As each of the positive electrode 104 and the negative electrode 105, for example, a porous conductive material such as carbon felt is used, and as the bipolar plate 121, for example, a flat plate made of a plastic carbon is used (Patent Literature 1).